1911-1920
Kokoro by Natsume Soseki
1921-1930
The Trial by Franz Kafka
1931-1940
Ali and Nino by Kurban Said
1941-1950
Cannery Row by John Steinbeck; everyone else can just stop nominating for this decade now

1951-1960
Rabbit Run by John Updike
1961-1970
Farewell Gul'sary by Chingiz Aitmatov
1971-1980 The Year of the Hare by Arto Paasilinna
1971-1980
If On a Winter's Night a Traveller by Italo Calvino
1981-1990
Hard-Boiled Wonderland and the End of the World by Haruki Murakami
1991-2000
A Suitable Boy by Vikram Seth
I haven't read enough from the first decade to make a reasonable nomination. It was a seriously difficult task in a couple of decades. Deliverance for 1961-1970 was close, A Fine Balance for 1991-2000 was close, there were at least five very good choices for 1981-1990.